A radio-frequency identification (RFID) tag itself does not have a battery and works relying on electromagnetic energy transmitted by a card reader. Owing to its simple structure, being economical and practical, RFID tags have been widely applied in the fields of logistic management, asset tracking, mobile health care and the like.
A radio-frequency front-end circuit of a passive RFID tag has two input ends which are respectively connected with two ends of an external inductance antenna coil that are also port-shared by a receiving end of the RFID tag that receives a downlink signal transmitted by the card reader and energy of a radio-frequency field, and a transmitting end of the RFID tag that outwardly transmits feedback uplink data signal to the card reader. Firstly, a passive RFID tag works by absorbing electromagnetic energy transmitted by the card reader from surrounding environment. After absorbing energy, the passive RFID tag rectifies a portion of energy into a DC power supply for a passive RFID tag's load circuit. Secondly, during an uplink communication process of transmitting data information from the RFID tag to the card reader, the passive RFID tag takes the form of load modulation, that is, controlling and changing the port impedance of a RF front-end by virtue of data information to be transmitted; the change of the port impedance leads to change of the current flowing in the inductance antenna coil of the passive RFID tag. A voltage waveform across both ends of the inductance antenna coil exhibits an amplitude modulated wave whose envelop magnitude changes according to the data being transmitted. Carrier frequency of the amplitude modulated wave is consistent with the carrier frequency of the RF field initiatively transmitted by the card reader, and the envelop magnitude of the amplitude modulated wave is related to the change of the port impedance, i.e., the envelop magnitude of the amplitude modulated wave is related to the data being transmitted. This amplitude modulated wave leads to change of the magnetic field induced by the inductance antenna coil. The change of the magnetic field is used as feedback data and received by the inductance antenna coil of the card reader through coupling effect of magnetic field, thereby completing the uplink data communication task.
Passive RF tag design is challenging in many aspects. A first challenge is to apply low-power circuit techniques to achieve complex functions of data transmission so as to satisfy battery-free design requirements. A second challenge is to apply low-cost design techniques to achieve as small as possible chip area so as to make package size small for high commercial profit. For example, an energy-storage capacitor is unavoidably necessary for a passive RF tag. The size of the energy-storage capacitor determines the available electrical energy in the circuit and is also a critical parameter that directly determines the circuit performance. In a modern deep sub-micron integrated manufacturing process, the size of capacitor is directly proportional to its occupied chip area without exception: the larger the chip area, the greater the energy-storage capacitor value, and the better the circuit performance. More importantly, a critical performance specification of a passive RF tag is its communication sensitivity, i.e. the maximum distance for the tag to perform reliable communication and various read-write operations; and the higher the sensitivity, the longer the distance, and the better the performance of the RF tag.
To tackle the above mentioned first challenge, a suitable technique is low-power system design on all aspects, including the design of system architecture, application of integrated circuit manufacture process that offers suitable integrated devices, exquisite design of circuit modules, optimization of physical layouts and the like. This is a wide and profound subject and is beyond scope of the present invention.
To tackle the above mentioned second challenge, it is essential to alleviate dependence of the prior art on a great number of energy-storage capacitors. The energy-storage capacitors are generally connected to an output of a low drop-out (LDO) voltage regulator circuit, which connects to the rectifier output. The circuit's switching among various modes, as well as modulating and demodulating digital commands cause instantaneous voltage pull-down fluctuation to various extents, i.e., abrupt change of the power supply voltage at the output of the LDO voltage regulator. A typical low-power LDO voltage regulator cannot sufficiently suppress these pull-down fluctuation through its internal error-correcting feedback loop. An only viable option is to increase the energy-storage capacitor connected with the output. Thus, the pull-down fluctuation problem is mitigated by transporting charges stored by the capacitor; and the larger the capacitor, the better the effect of suppression.
To cope with the above mentioned third challenge to improve communication sensitivity, the size of on-chip energy-storage capacitor is also much of concerned. The larger the energy-storage capacitor, the more the energy collected under the same distance condition, thus, the higher the sensitivity. Besides on-chip energy-storage capacitor, the factors directly related to sensitivity, or communication distance, are lies in the technology to realize the aforementioned load modulation. Load modulation changes the equivalent load impedance across the two ends of the antenna coil so as to change the current flowing the antenna coil and to further change the magnetic field generated by the current. The varying magnetic field is coupled to the antenna coil of a card reader, thereby completing the data transmission process. However, there is a limit of demodulation capability of the card reader. Due to the limited demodulation capability, only if the variation of the magnetic field be larger than a minimum amount can it be accurately demodulated at the card reader. Under constant coupling conditions, i.e., when communication distance is constant, variation of the magnetic field is determined by modulation depth on the RF tag's antenna coil. The modulation depth determines whether the card reader can accurately demodulate data signal. The greater the modulation depth, the easier the demodulation by the card reader, the longer the communication distance, and thus, the higher the sensitivity. The demodulation depth is generally defined as FIG. 1 in the industry. As an example, the value of each parameter of an amplitude modulated wave shown in FIG. 1 is shown in the following table.
Parameter symbolMinimumMaximumm = (a − b)/(a + b)90%100%TF14 * Tc10 * TcTF200.5 * TF1TF300.5 * TFd0X00.05 * aY00.05 * a
However, the modulation depth of the voltage on an RF tag's antenna coil decreases along with increase of energy of RF field. When the card reader is close to the RF tag, the passive RF tag is located in the strong RF field generated by the card reader. In this strong field environment, for reliability reasons of preventing internal devices of the RF tag from being broken down by excessively high voltage, peak value of the voltage amplitude is generally limited by an amplitude-limiting circuit. In this situation, due to the strong field, the wave trough is much higher than that in a weak field, and this crest-trough combination has resulted in insufficient modulation depth under the near and strong field condition. Because of this, a lot of low-cost card readers with a limited demodulation capability in the market cannot accurately demodulate uplink data transmitted by a RF tag. This is one of main reasons causing a communication response dead zone of an RF tag under near field condition which is frequently encountered in practice.
In prior arts a pull-down resistor is adopted to realize load modulation as shown in FIG. 2, i.e., a resistor with a constant resistance value is connected across the two ends of the RF tag's antenna coil after being serially connected with a switch device, wherein turning on and off of the switch device depends on the polarity of the data to be transmitted. For example, when the datum is “0”, the switch is turned on, and the pull-down resistor is connected across the two ends of the antenna coil to form a structure where the pull-down resistor is connected in parallel to the RF tag's equivalent impedance seen across the two ends of the antenna coil, thereby decreasing the coil impedance. Conversely, when the datum is “1”, the switch is turned off and the pull-down resistor does not influence the equivalent impedance seen across the two ends of the antenna coil. In order to make the pull-down more effective and to increase the demodulation depth, the resistance value of the pull-down resistor is expected to be smaller, but the excessively low trough under the weak field condition cannot meet the energy collection requirement. On the other hand, if the resistance value of the pull-down resistor is excessively large, pull-down under strong field condition will not be much effective, which leads to insufficient demodulation depth. Therefore, the value of the pull-down resistor for the load modulation in the prior art cannot give consideration to the performance requirements both under the strong field and the weak field conditions, and the RF tag cannot meet the market demands of high performance and low cost.
On the other hand, when an RF tag is far from a card reader, it is located in a weak field environment. Under such condition, the trough may decrease to a relatively low level, thereby making energy collection difficult. Because the oscillation amplitude on the antenna coil may be smaller than the sum of threshold voltages of all unidirectional conducting devices in the rectifier circuit during a wave-trough period of the RF signal's amplitude envelop, the rectifier stops working, and energy collection also stops, and the RF tag is only powered by the charges stored on the energy-storage capacitor. In the existing passive RF tag circuit structure shown in FIG. 3, in the period of wave-crest, the energy-storage capacitor is charged by virtue of the charges output by the rectifier, which also increase the voltage of the energy-storage capacitor and relevant nodes surrounding the energy-storage capacitor; and in the period of wave-trough, the voltages of the antenna coil ports may be lower than the voltage of the energy-storage capacitor and the relevant nodes surrounding the energy-storage capacitor. Under such condition, voltage of a substrate node of the unidirectional conducting unit in the rectifier circuit normally formed by an MOS device, if connected in the way that prior art normally does, may be higher than voltage of its source diffusion and its drain diffusion zone, thereby forming a forward conducting path. The charges on the energy-storage capacitor C1 may flow towards the antenna coil's port in accordance with the forward conducting current causing a loss of the energy-storage charges. Such a periodic loss influences the overall energy collection efficiency, so that the load circuit cannot work properly due to excessively low power supply voltage of the circuit and the RF tag has no response at the far end, i.e., the sensitivity of the tag is decreased.
In a pulse width modulated RF carrier signal, width of a wave-crest and a wave-trough of the signal envelop, i.e., the number of pulse periods of the carrier signal represents digit “1” or “0” to be transmitted (the polarity of the signals corresponding to the wave-crest and wave-trough is different if stipulated differently, which is not described in detail herein). For a transmission protocol with a low transmission rate, the wave-trough width may be greater than that of the transmission protocol with the higher transmission rate. Moreover, when the digital information (such as consecutive “0” digits) represented by a plurality of wave-trough pulse width signals is continuously transmitted, the wave-trough width may be very large, and the energy collection then faces great challenge.
Therefore, the application of energy-storage capacitor on passive RF tags directly determines two crucial performance parameters, namely cost and sensitivity of the passive RF tag. How to more efficiently utilize the energy-storage capacitor to maximize the sensitivity of an RF tag with a limited number of energy-storage capacitors becomes a focused research subject of the field.